The application cross-referenced above discloses and claims a method and apparatus for producing tomographic images. The images produced by the device described in the cross-referenced application can be considered to be the product of five terms, of which three are variable. Of the variable terms, one term is a function of the attenuation of the illuminating beam along its path from the source to the slice, another term is the proportion of incident energy which is scattered by the illuminated volume element of the object within the slice and the third term is a function of the attenuation of the scattered energy on its way out of the object being imaged. The two constant terms are the incident flux or illuminating radiation and geometrical factors dictated by the solid angle subtended by the detectors and the transmission ratio of the line collimator. An ideal image is formed when the only variable is the fraction of the illuminating energy which is scattered by the volume element being imaged. The fact that the image is also a function of the attenuation of the beam on its way in means that the resulting image is in a sense an image of the "slice" as seen through a filter (consisting of the overlying material) whose density is determined by the overlying material which is in line with the illuminating beam. Because the density of the overlying material can vary, those variations will show up as (unwanted) modulation in the resulting image, which are not the result of actual variations due to x-ray scattering within the "slice".
It is a purpose of this invention to eliminate or minimize this variable.
As described in the copending application, tomographic imaging in accordance with that invention is achieved by generating a pencil beam of illuminating radiation and sweeping that pencil beam over a line in space to define a sweep plane, locating the object to be imaged so that the pencil beam intersects the object and the selected slice, locating a line collimator whose line of focus coincides with that particular linear element of the selected slice which is illuminated by the sweeping pencil beam, e.g. locating the line collimator so that its field of view intersects the sweep plane in a bounded line which lies in the selected slice. A radiation detector is provided to respond to radiant energy passing the line collimator. For reasons more particularly described in the copending application, the radiant energy detector will "see" a sequence of signals describing the radiation response of that linear portion of the selected slice illuminated by the pencil beam. We can refer to this sequence as a line image of a linear element of the slice. The illuminated linear element or portion of the selected slice is the portion of the slice along the bounded line. By providing proper motion of the object relative to the source/detector/line collimator, we can provide for the bounded line to trace out the selected slice so that as a function of time, the response of the detector will describe the selected slice. Thus the image generated in the structure of the copending application is merely a concatenation of signal sequences (one or more sequence per line image) as described above, at least one sequence for each different instance of the bounded line in the selected slice.
It is an object of the present invention to reduce or minimize the modulation or variation of the tomographic image caused by variations in that portion of the object lying between the selected slice and the source of the pencil beam. As described in the copending application, density variations in this portion of the object will "modulate" the resulting tomographic image. Thus it is an object of the invention to reduce or minimize this "modulation" of the resulting tomographic image.